I will touch upon just one more topic before we get on with the khayal performance. And that is the matter of scales: Natural and tempered.
The keys of the piano (or even a harmonium) look as shown below. The base octave is played with 12 keys, consisting of 7 white and 5 black ones. The keys are denoted from C to B as shown. These octaves would be repeated one after the other, starting from a very low register to higher and higher.
Western Music, as we saw earlier, is symphony based, which means a lot of instruments would be playing at the same time. Also, the base note could keep shifting within the composition. This necessitates standardization of different pitches, with absolute frequencies defined. This also poses another requirement within the octave, in that the distances between all the successive notes have to be equal. This is possible only if the scale is divided into twelve equal parts from the Sa to the taar Sa. This is exactly what is done, and is known as tempering of the scale. In this scale, you can start with any note as the base sa, and play the whole octave. For standardization, the A key of the middle octave is tuned to a frequency of 440 Hz. All the rest of the 11 keys would just follow mathematically.
Consider Indian music on the other hand. If we imagine the C to be the Sa, then the white keys would all play shuddha swara, while the black ones would be the komal swaras and the teevra madhyam, giving the entire saptak. The natural scale would have the exact positions of the shuddha swara arrived at through the harmonics of the Sa. These positions do not coincide with those of the tempered scale, and there are variances, ranging from very minor to considerable from note to note. Another problem is, the komal swara positions are raga specific, and could be many within the space between two shuddha swara.
Till the harmonium emerged onto the scene about a century ago, sarangi was the only accompanying instrument, besides the tanpura. This posed no problem at all, as the sarangi could be tuned to any note, and can also produce any notes within the continuum of the whole saptak. The singer thus can tune the tanpura and the harmonium to any point of his convenience, tune the tabla to the same note, and go!
With the harmonium, however, it became necessary to tune the tanpura to any one particular key. The singers hence started denoting their scales in terms of the keys of the harmonium, such as a safed 2 or a black 5 etc. In the last few years, however, with the electronic tanpuras and the software appas around, present day musicians have started using the western language of denoting the scale in terms of a C# or an A etc. Further, the western octave is divided into 1200 equal parts, called cents. This makes the distance between any two successive keys equal to 100 cents. So we now have a unit for fine tuning in terms of cents, e.g. 20 cents above E etc.
Tempering was done about three centuries ago, and our ears have become accustomed to the approximation, which was particularly easy in case of symphonic music. However, even within western classical music, the symphonies are composed for and always played in a particular key, where they sound the best.
For raga sangeet however, which is purely monophonic, the tempered scale does not work. An Indian classical musician is constantly in pursuit of becoming one with the natural harmony, and relies completely on the tanpura for his guide. The harmonium used for vocal accompaniment tries hard to meet up with special tuning of the instrument for particular base notes, and we have come to accept the approximation over the last century.